Foundry mold coating



United States Patent 3,115,414 FBUNDRY MOLD COATlNG Neil M. Lottridge, .lr., Warren, and Douglas G. McCullougll, Rochester, Mich, assignors to General Motors (Iorporation, Detroit, Mich, a corporation of Delaware No Drawing. Filed Nov. 28, 1960, Ser. No. 71,903 7 Claims. (Cl. 10638.23)

This invention relates to a coating composition which, when applied to foundry sand mold and core surfaces, significantly increases the fluidity of molten casting metals, such as aluminum alloys, magnesium alloys and ferrous metals, poured into contact with the coated mold or core. More specifically, the invention pertains to a foundry sand mold or core wash containing finely pulverized mica and wood flour suspended in a liquid carrier.

in the past it was possible to successfully cast many thin sect-ions of gray iron, malleable iron, spheroidal graphite cast iron, aluminum alloys and magnesium alloys only when the casting metal was superheated to a pouring temperature appreciably above that conventionally used, the mold was preheated and/ or the metallostatic pressure was substantially increased. It obviously is desirable to eliminate these procedures. A principal object of the present invention, therefore, is to provide a coating for foundry sand molds and cores which materially increases the fluidity of molten casting metals contacting such molds and cores and which thus eliminates the necessity of superheating these metals, preheating the molds or raising the Inetallostatic pressures to an undesirable extent. As a consequence, sound castings of very thin section can be formed by conventional casting procedures. The term fluidity, as used herein, refers to the property of molten casting metal in contact with a mold which allows the metal to flow into the mold and fill it before solidification obstructs further flow of the metal.

The above and other objects of this invention are attained with a foundry mold and core coating comprising a slurry of mica or vermiculite powder, wood flour and a liquid vehicle, such as methyl alcohol, ethyl alcohol, isopropyl alcohol, acetone, toluene, mineral spirits and water, to which a small amount of low-foaming, nonionic wetting agent preferably has been added. For many ap plications it is desirable to include pitch, such as coal tar pitch, and a lower alkyl cellulose, preferably methyl cellulose, in the coating slurry. The mold coating thus provided is relatively inexpensive and can be readily applied by conventional means, such as spraying, dipping or brushmg.

It will be understood that the term mold, as hereinafter generally employed, means a casting form which includes both molds and cores, this invention not being limited to the former. Likewise, the word sand is used in its generic sense and is not restricted to silica particles.

The mica powder should constitute about 50% to 85% by weight of the solids content of the coating composition, While the amount of wood flour to be used may range from about 2% to 50% of the weight of the solids. Normally, however, the amount of wood flour will not exceed approximately 25%. When the quantity of wood flour present is below approximately 15% by weight, sufficient pitch should be added to raise the total amount of wood flour plus pitch to at least 15%. In general, the maxi mum pitch content should not exceed about 25% of the weight of the solids in the coating composition, and at least 5% pitch is useful for many applications.

The coating slurry preferably is applied to green sand molds by a spraying procedure, and when this is done it 3,115,414 Patented Dec. 24, 1963 acetone and toluene. A slurry of this type also may be used to spray baked sand molds and cores, or water may be used as a less expensive carrier for spraying such molds or cores if they can be subsequently reheated to dry the coating.

In order to obtain a proper coating When a water suspension is used in a dipping operation, it is desirable to include a wetting agent, water thickener, and/ or binder in the slurry. A low-foaming, nonionic wetting agent is preferred. Methyl cellulose has proved to be the most satisfactory of the water thickeners, but other shortchained alkyl celluloses, such as ethyl cellulose, butyl cellulose and propyl cellulose, may be employed. A nonionic, etheriiied polyarnide carbohydrate which is a water-soluble derivative of wheat starch also has. proved to be an excellent suspension agent and binder. A water thickener of this type appreciably improves the dipping characteristics of the slurry and increases the hardness and durability of the dried coating.

The amount of the carrier used should be adequate to reduce the viscosity of the slurry to a point where it may be applied readily. if it is desired to dip or spray the mold, it usually is advantageous to include a relatively high percentage of liquid carrier in the coating composition. in general, the amount of liquid in the slurry may vary from about 50% to 95% by weight, and excellent castings have been obtained with a coating having a water content of approximately 60% to 90%. This amount of liquid vehicle appears to produce optimum results with respect to dipping characteristics, drying time of the slurry, and hardness and durability of the dried coating.

When the dry constituents in the coating composition are mixed with the liquid vehicle in the aforementioned preferred proportions, the resultant slurry will contain approximately 5% to 30% by weight of mica powder and 0.2% to 15% by weight of wood flour. If pitch is included in the mix, this component normally will constitute about 0.5% to 6% of the total weight of the slurry. In general, the particle sizes of these powdered constituents should be between and 325 mesh, although liner powders may be used.

When a wetting agent is used in the mixture, the addition of 1 gram to 3 grams of this material to 500 milliliters of water in the slurry has proved to be adequate. In fact, as little as about 0.1% by weight of wetting agent improves the coating composition, and as much as 1% wetting agent may be used to advantage. The amount of methyl cellulose to be used may range between approximately 0.2% and 1% of the total weight of the coating composition to provide the aforementioned improvements in dipping characteristics and mechanical properties of the mold coating.

In addition to the above-described constituents, urea formaldehyde also may be included in the slurry, particularly in combination with a wetting agent, to further increase the hardness and durability of the dried coating. A relatively small amount of other binder materials also may be added to the slurry to increase the adhesion of the coating to the sand mold if the binder is soluble in the liquid carrier. Among the binding agents which are recommended are thermoplastic resins such as polyvinyl chloride, vinylidene chloride, polyvinyl alcohol and polyvinyl acetate, and thenmosetting resins such as phenolfo-rmaldehyde and melamine formaldehyde. Of course, various other binders besides the polymeric vinyl and phenolic resins can be employed. For example, methyl cellulose, linseed oil, corn cereal and certain clays such as bentonite clay are examples of substances which are useful as binder constituents.

When a coating containing mineral spirits or alcohol as the carrier is to be applied by spraying, a small amount of certain thermoplastic, lignin-type resinous materials derived from wood have proved to be desirable binders. The molecules of these resins contain both methoxyl and hydroxyl groups. Such materials may be obtained from a variety of 'wood products including sawdust, waste liquor from .the paper industry, wood chips, etc, by physical and/or chemical treatment. Illustrative of one type of thermoplastic resinous material is a substantially gasolineinsoluble resinous material obtained by extracting a resinous wood with a coal tar hydrocarbon, removing said hydrocarbon by evaporation, leaving a residue comprising a mixture of wood rosin and thermoplastic resinous material, and extracting the rosin with a petroleum hydrocarbon, leaving a thermoplastic resinous material.

Of course, more than one binder may be used in the coating composition, but the total binder content normally should not constitute more than about 15% of the weight of the solids in the coating material. Even 0.5 by weight of binder in the coating causes it to adhere more strongly to the mold. In the case of urea formaldehyde, this component may be added to the slurry in an amount as high as about 25% by weight of the solids content of the coating composition.

When a lower alkyl cellulose is included in an aqueous suspension of the coating material, we have found it desirable toinclude a small amount of an anti-fermenting agent. Copper sulfate, potassium chromate, sodium chromate, urea :formaldehyde, paraiormaldehyde and hydrogen peroxide all have proved to be useful for this purpose. Shelf life of an aqueous slurry has been increased from approximately 3 days to more than 2 weeks because of the anti-bacterial action of such an additive.

The above-described coating slurry may be prepared in the following manner: The dry materials, such as the mica, wood flour, methyl cellulose and pitch, are first blended to powder form. Next, the wetting agent is added to the desired amount of water, assuming an aqueous slurry is to be used; and the blended solids are introduced slowly into the slurry while the latter is being agitated. If urea formaldehyde is to he included in the coating, it is the last constituent added to the slurry. The coating composition normally is applied to the casting-defining surfaces of the mold at room temperature.

The following table lists specific examples of coating compositions prepared in accordance with the present invention:

Example Example Example Example I II III IV Water 500 ml 500 ml..." 500 ml".-. Mineral Spirits Mica Powder (-160 mesh).

Pitch (-120 mesh and 200 mesh).

Wood Flour (-80 mesh).

Wetting Agent Polyvinyl Acetate (50% water solution).

Urea Formaldehyde Methyl Cellulose..-

Isopropyl AlcohoL.-.

35 gels. 57 lbs.

14 lbS.

3 lbs.

50 gm.. 30 gm 50 gm 125 gm 10 gm 12.5 gmm.

2.5 gm 1 gm 2.5 gm

5.5 lbs.

In Example IV the isopropyl alcohol functions as an emulsifier for the polyvinyl acetate binder solution.

Each of the above coating compositions substantially reduced chill in thin sections and materially aided in the cleanup of the castings. Furthermore, use of these coatings as compared with currently commercially used green sand mold sprays and core washes significantly decreased metal penetration into the mold and/or cores, thereby improving the surface finish f the castings in their as-cast condition.

When drysand test molds were coated with the abovedescribed slurries, the fluidity of gray cast iron, malleable iron and spheroidal graphite cast iron poured at normal casting temperatures into these molds was increased approximately as compared with similar molds which were uncoated. With green sand test molds, the fluidity increase was more than 100%. The fluidity of the molten metals in the molds was determined by means of a double spiral fluidity test mold in which one spiral is coated with the mold wash wlii e the other remains uncoated, both being fed -'by the same pouring basin and down sprue. This test mold design and the procedure for using it are described in Transactions of the American Foundrymens Society, volume 67 (1959), pages 496 to 507.

Tests have been .conducted on castings of .gray iron, malleable iron, spheroidal graphite cast iron, magnesium allows and aluminum alloys which were formed in molds coated in accordance with this invention. As a result of these tests, it was found that these metals could be successfully cast in thin sections by using the above-described mold wash and that the resultant castings all had exceptionally high ultimate tensile strength and hardness.

For example, As-inch thick sections of gray iron cast in green sand mold portions coated with this mold Wash were compared with gray iron specimens of the same composition from li -inch thick sections cast in the same molds without a coating. The cast iron in the As-inch thick sections had appreciably higher ultimate tensile strength and hardness than the cast iron in the A -inch thick sections. Attempts to cast /s-inch thick sections without first coating the surfaces of the mold cavity were unsuccessful because the molten metal would not properly feed into and fill the narrow mold cavities. The gray cast iron which did feed into these /s-inch cavities of uncoated molds was chilled, and hence these portions of the castings were hard, brittle and unmachinable. Green sand molds, each having two cavities four inches wide and 18 inches long, with sections /s-inch and inch deep, were used in these tests. Test bars with a section 0.90 inch thick x050 inch wide and a gage length of two inches were machined from each slab casting of gray iron.

It was further noted that the microstructure of the Aa-inch thick sections of gray iron castings poured into the coated mold portions were slightly more refined than the microstructure of gray iron specimens cast into the yi inch thick uncoated portions of the same mold. No primary carbide was evident in either section.

Additional tests under production conditions also showed that use of the new mold coating improved the surface finish of the castings and decreased the incidence of veining, thus greatly reducing the amount of labor necessary in cleaning operations. Moreover, the coating prevented misruns and cold shuts. As a consequence, leakers were eliminated, and the scrap rate due to such defects was lowered from approximately 9% to zero. Furthermore, the mold coating described herein has a relatively short drying time and a wide drying temperature range. It may be dried at a temperature up to approximately 500 F. without blistering, and coatings on port cores for gasoline engine cylinder head castings were dried in only about five minutes at 500 F. This coating also has good dipping characteristics and is uniform in thickness both before and after drying, as well as being relatively inexpensive to use.

While our invention has been described by means of certain specific examples, it is to be understood that its scope is not to be limited thereby except as defined by the following claims.

We claim:

1. A slurry for coating casting-defining surfaces of foundry sand molds, said slurry consisting essentially of a dry blend of approximately 50% to 85% by Weight of mica powder, 2% to 5 0 by weight of wood flour and 5 to 25 by weight of pitch to which has been added liquid carrier in an amount sufficient to constitute about 50% to of the total weight of the slurry.

2. A coating slurry for application to casting-defining surfaces :of a foundry sand mold, said slurry consisting essentially of about to 30% by weight of mica powder, 0.2% to by Weight of WOOd flour, 0.5% to 6% by weight of pitch, 60% to 90% by Weight of at least one liquid vehicle selected from the group consisting of methyl alcohol, ethyl alcohol, isopropyl alcohol, ace-tone, toluene, mineral spirits and water, and 0.2% to 1% by weight of a louver alkyl cellulose.

3. A coating slurry for application to casting-defining surfaces of a foundry sand mold, said slurry consisting essentially of about 5% to by Weight of mica powder, 0.2% to 15% by Weight of wood flour, 0.1% to 1% by weight of wetting agent, 0.5% to 6% by weight of finely divided coal tar pitch, to 90% by Weight of water, and 0.2% to 1% by weight of a lower alkyl cellulose, the particle sizes of said mica powder, wood flour and pitch not exceeding approximately mesh.

4. A foundry mold for use in pnoducing a metal casting having at least one thin section, said mold being formed of foundry sand having .castingdefining surfaces provided with a thin coating consisting essentially of about 50% to by weight of mica, 2% to 50% by weight of Wood flour, and pitch not in excess of 25% by weight.

5. A foundry mold for use in producing a metal casting having at least one thin section, said mold being formed of foundry sand having casting-defining surfaces provided with a thin coating consisting essentially of about 50% to 85% by weight of mica powder, 2% to 5 0% by weight of Wood flour and 5% to 25 by Weight of coal tar pitch.

6. A method :of preparing a coated foundry sand mold which permits molten casting metal in contact with said mold to more readily flow into the mold cavity and fill it before solidification obstructs further flow of said metal, said method comprising applying to casting-defining surfaces of said mold a coating slurry consisting essentially of about 5% to 30% by Weight of mica powder, 0.2% to 15% by weight of Wood flour, 60% to by weight of at least one liquid vehicle selected from the group consisting of methyl alcohol, ethyl alcohol, isopropyl alcohol, racetone, toluene, mineral spirits and water, and 0.2% to 1% of a lower alkyl cellulose.

7. A method of preparing a coated foundry sand mold which permits molten casting metal in contact with said mold to more readily flow into the mold cavity and fill it before solidification obstructs further flow of said metal, said method comprising applying to casting-defining surfaces of said mold a coating slurry consisting essentially of about 5% to 30% by weight of mica powder, 0.2% to 15 by Weight of wood flour, 0.5% to 6% by weight of pitch, 60% to 90% by weight of at least one liquid vehicle selected from the group consisting of methyl alcohol, ethyl alcohol, isopropyl alcohol, acetone, toluene, mineral spirits and water, and 0.2% to 1% by weight of a lower alkyl cellulose.

References Cited in the file of this patent UNITED STATES PATENTS 2,430,655 Wallace Nov. 11, 1947 2,622,963 Wallace Dec. 23, 1952 2,735,814 Hodson et al. Feb. 21, 1956 2,798,817 Lund July 9, 1957 

1. A SLURRY FOR COATING CASTING-DEFINING SURFACES OF FOUNDRY SAND MOLDS, SAID SLURRY CONSISTING ESSENTIALLY OF A DRY BLEND OF APPROXIMATELY 50% TO 85% BY WEIGHT OF MICA POWDER, 2% TO 50% BY WEIGHT OF WOOD FLOUR AND 5% TO 25% BY WEIGHT OF PITCH TO WHICH HAS BEEN ADDED LIQUID CARRIER IN AN AMOUNT SUFFICIENT TO CONSTITUTE ABOUT 50% TO 95% OF THE TOTAL WEIGHT OF THE SLURRY. 